1. Field of the Invention
The invention relates to an apparatus for making a droplet target provided with at least one receptacle for receiving a target liquid and in which high pressure is generated by means of gaseous nitrogen, a magnetic valve connected to the receptacle and switchable in the ms range, and a nozzle, as well as to a method of forming a droplet target.
2. The Prior Art
Hereafter, devices known in the prior art will be described by which liquid droplets are being generated wherein the interaction of laser beams aimed at these droplets generates X-rays or extreme ultra-violet light. Such rays are used, for instance, in microscopy and lithography.
U.S. Pat. No. 6,324,256 describing an arrangement of a laser plasma source for generating EUV light, also refers to a device for making droplet targets. The droplets made are of a diameter larger than the diameter of droplets generated by a gas fed through a nozzle where it condenses to form a cloud of clusters of extremely small particles. As described, a liquid is formed from the gas by means of a heat exchanger which reduces the temperature of the gas. The liquid is fed to a nozzle the opening of which increases in the direction of the exit opening. The droplets are formed in this section and then exit from the exit opening of the nozzle to interact with a laser beam for generating EUV light. However, it is not possible in this arrangement in a defined manner to set the size of the droplets. In this arrangement the gaseous initial material is converted to a liquid one. Moreover, the droplets interact with the laser very close to the nozzle which in consequence of the heat and erosion is destroyed.
In Opt. Common. 103, 105 (1993), L. Ramble and H. M. Hertz report on an X-ray source in which droplets of ethanol are used as the target. To generate these droplets, ethanol was pressed at 30 to 50 at into a vacuum chamber through a capillary of about 10 μm diameter tapering in the direction of the nozzle. In order to generate a liquid volume—in this case of a diameter of 15 μm—pressure surges were piezo-electrically produced at a frequency of about 1 MHz. The relatively large droplets were used for examining the interaction with laser radiation in an intensity range of 1012 to 1014 W/cm2 as described by O. Hemberg, B. A. M. Henson, M. Berlund and H. M. Hertz in J. Appl. Phys. 88, 5421 (2000). Since in this case each individual droplet is interacting and the laser focus is but slightly larger than the diameter of the droplets of ethanol, the drift problem of the droplet source is of major importance, the project is especially directed to solving an exact droplet-laser synchronization.
Super dense droplet spray of a density of up to 1019 atoms/cm3 and a droplet diameter of about 1 μm was produced by a droplet source described by L. C. Mountford, R. A. Smith and M. R. H. R. Hutchinson in Rev. Sci. Instrum. 69, 3780 (1998) and is the basis of the instant invention. The basis of this droplet source is a magnetic valve which forms the pulse of liquid and, therefore, the volume of the liquid. A receptacle was filled with a liquid and kept under high pressure by means of methanol. The valve is opened in synchronism with the laser pulse and for 2,500 μs to allow droplets to emerge from the nozzle. It was possible to produce droplets of lesser diameter of about 0.6 μm by subsequent electrostatic cleaving of the droplets. This, however, requires a technically complex arrangement. However, the jog consisting of such droplets is of lower density, viz. about 1016 atoms/cm3.
For effectively generating X-rays or EUV light it is necessary, however, to make available droplet targets of dimensions of the size of possible laser wavelengths (T. D. Donelly, M. Rust, I. Weiner, M. Allen, R. A. Smith, C. A. Steinke, S. Wilks, J. Zweiback, T. E. Cowan, and T. Ditmire, J. Phys. B: At. Mol. Opt. Phys. 34, L313 (2001)) and, therefore, of a smaller diameter compared to the prior art, and which form a spray of an atomic density of >1018 atoms/cm3.